The present invention concerns a method and an apparatus for controlling system parameters, in particular for controlling the voltage applied to piezoelectric elements within a circuit for charging and discharging piezoelectric elements.
Control systems generally comprise a control unit typically but not necessarily comprising a central processing unit (CPU), at least one controlled element and utilization means which transform CPU signals if and as necessary and apply them to the controlled element. For this purpose, the CPU and the utilization means need to be connected to each other by communication means such as a bus system. Moreover, external data may need to be communicated to the CPU and/or the utilization means on a corresponding way.
As an example, piezoelectric elements may be used as actuators in fuel injection nozzles (in particular in so-called common rail injectors) of an internal combustion engine. The use of piezoelectric elements with double acting, double seat valves to control corresponding injection needles in a fuel injection system is shown in German patent applications DE 197 42 073 A1 and DE 197 29 844 A1, which are incorporated by reference herein in their entirety.
Fuel injection systems using piezoelectric actuators are characterized by the fact that, to a first approximation, piezoelectric actuators exhibit a proportional relationship between applied voltage and the linear expansion. In a fuel injection nozzle, for example, implemented as a double acting, double seat valve to control the linear stroke of a needle for fuel injection into a cylinder of an internal combustion engine, the amount of fuel injected into a corresponding cylinder is a function of the time the valve is open, and in the case of the use of a piezoelectric element, the activation voltage applied to the piezoelectric element.
FIG. 5 is a schematic representation of a fuel injection system using a piezoelectric element 2010 as an actuator. Referring to FIG. 5, the piezoelectric element 2010 is electrically energized to expand and contract in response to a given activation voltage. The piezoelectric element 2010 is coupled to a piston 2015. In the expanded state, the piezoelectric element 2010 causes the piston 2015 to protrude into a hydraulic adapter 2020 which contains a hydraulic fluid, for example fuel. As a result of the piezoelectric element""s expansion, a double acting control valve 2025 is hydraulically pushed away from hydraulic adapter 2020 and the valve plug 2035 is extended away from a first closed position 2040. The combination of double acting control valve 2025 and hollow bore 2050 is often referred to as double acting, double seat valve for the reason that when piezoelectric element 2010 is in an unexcited state, the double acting control valve 2025 rests in its first closed position 2040. On the other hand, when the piezoelectric element 2010 is fully extended, it rests in its second closed position 2030. The later position of valve plug 2035 is schematically represented with ghost lines in FIG. 5.
FIGS. 6a, 6b, and 6c show the double acting control valve in the first closed position, open position, and in the second closed position respectively.
The fuel injection system comprises an injection needle 2070 allowing for injection of fuel from a pressurized fuel supply line 2060 into the cylinder (not shown). When the piezoelectric element 2010 is unexcited or when it is fully extended, the double acting control valve 2025 rests respectively in its first closed position 2040 or in its second closed position 2030, as shown in FIGS. 6a and 6c, respectively. In either case, the hydraulic rail pressure maintains injection needle 2070 at a closed position. Thus, the fuel mixture does not enter into the cylinder (not shown). Conversely, when the piezoelectric element 2010 is excited such that double acting control valve 2025 is in the so-called mid-position with respect to the hollow bore 2050, then there is a pressure drop in the pressurized fuel supply line 2060. This situation is illustrated in FIG. 6b. This pressure drop results in a pressure differential in the pressurized fuel supply line 2060 between the top and the bottom of the injection needle 2070 so that the injection needle 2070 is lifted allowing for fuel injection into the cylinder (not shown).
Within the example of FIG. 7, the use of having two open positions resp. two voltages corresponding to open positions is as follows: Generally, it may be desired that the first and second open position of the valve plug 2035 within the hollow bore 2050 are equal to each other. However, in order to achieve this aim it has to be taken into account, that in one case the open position is approached from the first or bottom closed position and in the other case from the second or top closed position. While recalling FIG. 6a through FIG. 6c, this means, that in the first case the valve plug 2035 is moved against a force applied by the pressure prail in the pressurized fuel supply line 2060 and in the second case it is moved with the force applied by the pressure prail in the pressurized fuel supply line 2060. Hence, in the first case a larger force is to be applied than in the second case. Since the valve 2025 is driven by the piezoelectric element 2010, this again means, that in the first case a higher voltage is required than in the second case in order to obtain the desired expansion of the piezoelectric element and hence the desired open position of the valve plug 2035.
In addition, in the above example, the movement of the double-acting valve and hence of the injector needle is influenced by any changes of the occurring pressure prail, since so far it is only taken into account, that different voltages are required in order to similarly position the valve plug 2035 while moving it against and with the pressure prail. However, so far it is not taken into account what further modifications are required in order to deal with changing pressures prail which occur in practice. In order to nevertheless control the movement of the injector needle with high precision respectively to control the corresponding amount of injected fuel with high precision these influences have to be taken into account. Hence, the occurring rail pressures are measured by measuring means and the target voltages which are to be applied to the piezoelectric elements for desired actions are modified in a corresponding way. As a result, a feedback system is implemented, in which rail pressures prail are measured by measuring means, the measured values are communicated to the control unit, corresponding target voltages for the piezoelectric actuators are calculated within the control unit (for example by means of adding an offset to a more general target voltage) and are communicated from the control unit to an utilization unit, for example an activation IC, from which they are applied to the piezoelectric actuators.
It is to be understood, that the above examples are introduced only in order to allow a better understanding of the inventive approach However, any further environment, particularly having a single-acting control valve or using a double-acting control valve in a single acting mode (i.e. temporarily or permanently not using the second or top closed position of the valve plug 2035, for example due to very cold temperatures which may cause difficulties to a full expansion of the piezoelectric element 2010) may be an environment for the present invention without any limitations. Furthermore, any other driving profile than the one mentioned above may be used. Moreover, control systems having a totally different technical background may be an environment for the present invention as well.
In the control system of above examples as well as in other control systems, there is a need for obtaining a good correspondence of the target values and the obtained values of each control procedure in order to achieve a high precision of the performance of the controlled system. However, this is limited due to properties of each individual control system as according to the state of the art.
It is an object of the present invention, to provide an improved method and apparatus for controlling system parameters.
This object of the present invention is achieved by a method for controlling system parameters, in particular for controlling the voltage applied to piezoelectric elements within a circuit for charging and discharging piezoelectric elements. At least one control parameter for the control of a system parameter, in particular a target voltage for the voltage applied to a piezoelectric element is modified in view of at least one systematic error occurring during a first control procedure of the system parameter to obtain a corrected control parameter for a second and/or a further control of the system parameter.
An object of the present invention is further achieved by an apparatus, in particular eligible for usage with the inventive method, in which are implemented: modification means for the modification of system parameters according to at least one control parameter; measuring means for the measurement of the value of the resulting system parameter; comparison means for the comparison of the measured value to a predefined target value; and calculation means for the calculation of at least one control parameter for a further modification of the system parameter in accordance with differences occurring between the measured value and the target values.
As stated, the general approach of the invention is to improve the performance of the controlled system during future control procedures rather than during a first present control procedure (hence, as an example, within a first charging procedure a target voltage of 100 V is used in order to bring a piezoelectric element to a voltage of 100 V; however, after termination of the procedure, it turns out that in fact just a voltage of 90 V was achieved; hence, the next time when an element should be brought up to 100 V, a corrected target voltage of 110 V is used, since the foregoing experience teaches that there is a minus of 10 V while doing so; similarly, in case one element should be brought to a target of 150 V a corrected target of 160 V may be used and so on). Hence, an existing control system, in particular a system for controlling the voltage applied to piezoelectric elements within a circuit for charging and discharging piezoelectric elements, can be used without any (or without any significant) technical modifications of the system, and the performance is nevertheless improved for the major part of the control procedures. This in particular holds in case of a large number of control procedures during a typical application of the control system. Hence, the invention provides an inexpensive and elegant improvement of the performance of a control system.
Advantageous implementations of the invention are addressed. Corresponding implementations provide the advantage of modifying the desired voltage in order to substantially eliminate the systematic errors that occur within the control system and result in inaccurate control of the piezoelectric actuators.